A power steering device is in wide use as a device that reduces a force which is required for a driver's steering wheel operation when a steering angle is applied to a steering wheel (usually a front wheel except for the case of special vehicles such as forklifts). Electric power steering devices that use an electric motor as an auxiliary power source in such power steering devices are increasingly utilized as of late, too. The electric power steering device can be more compact in size and lighter in weight than a hydraulic power steering device and controlling a magnitude (torque) of auxiliary power is easier with the former than the latter. The other advantages of the electric power steering device include less engine energy loss.
Various structures have been known as the electric power steering device. In every case, the auxiliary power of the electric motor is applied via a speed reducer to a rotating shaft for steering that is rotated by the steering wheel operation and applies the steering angle to the steering wheel as a result of the rotation. In general, a worm-type speed reducer is used as the speed reducer. In the case of the electric power steering device that uses the worm-type speed reducer, the auxiliary power of the electric motor can be transferred to the rotating shaft for steering when a worm driven to rotate by the electric motor and a worm wheel rotating with the rotating shaft for steering mesh with each other.
As an example, a patent document 1 discloses the electric power steering device that is illustrated in FIGS. 10 and 11. In this electric power steering device, a front edge of a steering shaft 2 that is a rotating shaft for steering which is rotated in a predetermined direction by a steering wheel 1 is supported to be rotatable inside a housing 3 and a worm wheel 4 is fixed to the front edge of the steering shaft 2. In a state where the worm wheel 4 meshes with worm teeth 5 disposed in an axial intermediate portion of a worm shaft 6 driven to rotate by an electric motor 7, a base edge of the worm shaft 6 and a tip portion of the worm shaft 6 are rotatably supported inside the housing 3 by a base end side bearing 8 and a tip side bearing 9, respectively.
In most cases, the worm-type speed reducer that is based on the meshing of the worm wheel 4 and the worm teeth 5 has an inevitable backlash in a meshing portion of the worm wheel 4 and the worm teeth 5. This backlash results from an abrasion of a tooth surface of the worm wheel 4 and the worm teeth 5 as well as a dimensional error and an assembly error of each member constituting the worm-type speed reducer. In recent years, in particular, the amount of the abrasion is on the increase and the backlash is becoming more and more likely to occur as the auxiliary power tends to be increased. When the backlash is present in the meshing portion by any means, a jarring rattling noise might be generated in the meshing portion when the direction of the rotation of the steering shaft 2 is changed and when rotational vibration is applied from a wheel side to the steering shaft 2.
In the case of the structure that is illustrated herein, the worm teeth 5 are biased toward the worm wheel 4 by the worm shaft 6 being allowed to oscillate about the base end side bearing 8 such that the backlash in the meshing portion of the worm wheel 4 and the worm teeth 5 is removed.
For this reason, in the case of the structure that is illustrated herein, a holding recessed portion 10 is disposed at a part around the tip portion of the worm shaft 6 inside the housing 3 and a holder 11 is held and fixed inside the holding recessed portion 10. An outer ring constituting the tip side bearing 9 is internally fitted into and fixed to the holder 11 and an annular bush 12 formed of an elastic material is internally fitted into and fixed to an inner ring constituting the tip side bearing 9. A near-tip part of the worm shaft 6 is supported to be rotatable with respect to the holder 11 and be capable of a perspective motion with respect to the worm wheel 4 by the near-tip part of the worm shaft 6 being loosely inserted into the bush 12. At a part inside the holding recessed portion 10 that is adjacent to an axial outside of the holder 11 (right side in FIG. 11), a preload pad 13 is disposed to be capable of a displacement in relation to a meshing direction of the worm wheel 4 and the worm teeth 5 (vertical direction in FIG. 11). The tip portion of the worm shaft 6 is inserted into a through-hole disposed in a central portion of the preload pad 13 to be capable of relative rotation with respect to the preload pad 13 and without rattling in a radial direction. The tip portion of the worm shaft 6 is pressed toward the worm wheel 4 via the preload pad 13 by elastic force of a coil spring 14 laid across the preload pad 13 and the holder 11. In this manner, the worm teeth 5 are biased toward the worm wheel 4 by the worm shaft 6 being allowed to oscillate about the base end side bearing 8, and thus the backlash of the meshing portion of the worm teeth 5 and the worm wheel 4 is suppressed and the generation of the rattling noise in the meshing portion is suppressed.
As described above, in the case of the electric power steering device that is illustrated in FIGS. 10 and 11, the preload pad 13 is disposed inside the holding recessed portion 10 to be capable of the displacement relating to the meshing direction of the worm wheel 4 and the worm teeth 5 (vertical direction in FIG. 11), and a minute guide gap relating to a direction perpendicular to each of the meshing direction and an axial direction of the worm shaft 6 (front and rear direction in FIG. 11) is disposed between the preload pad 13 and a stationary member present therearound such that the displacement in the meshing direction is smoothly performed. Accordingly, inside the holding recessed portion 10, the preload pad 13 can also be displaced in the perpendicular direction (front and rear direction in FIG. 11) by the same amount as the guide gap. As a result, the worm teeth 5 can also be displaced in the perpendicular direction with respect to the worm wheel 4.
A meshing reaction force that is applied to the worm shaft 6 from the meshing portion of the worm wheel 4 and the worm teeth 5 includes not only a component in the meshing direction (vertical direction in FIG. 11) but also a component in the perpendicular direction (front and rear direction in FIG. 11). This point will be described below with reference to FIGS. 12 to 14.
As illustrated in FIGS. 12 to 14, the meshing reaction force is applied to the worm shaft 6 from the worm wheel 4 when a driving force is transferred from the worm shaft 6 to the worm wheel 4 by the worm shaft 6 being driven to rotate. In the case that is illustrated in FIG. 12 and the case that is illustrated in FIG. 13, the driving forces that are applied to the worm shaft 6 are equal to each other in magnitude but the driving forces that are applied to the worm shaft 6 have opposite directions of rotation. Accordingly, the worm wheel 4 rotates in the opposite directions in the case that is illustrated in FIG. 12 and the case that is illustrated in FIG. 13. In this state, apparent meshing reaction forces that have Fx, Fy, and Fz components of force, which are components in three respective directions of x, y, and z illustrated in FIGS. 12 and 13, are applied from the worm wheel 4 to the worm shaft 6 in the meshing portion of the worm wheel 4 and the worm teeth 5. Among these components of force Fx, Fy, and Fz, Fx and Fz are opposite in direction in the case of the rotation of the worm wheel 4 in one direction illustrated in FIG. 12 {direction illustrated by an arrow A in FIG. 12A} and in the case of the rotation of the worm wheel 4 in the other direction illustrated in FIG. 13 {direction illustrated by an arrow B in FIG. 13A}.
In a case where a distance between the meshing portion and an oscillation center o of the worm shaft 6 relating to the radial direction of the worm shaft 6 is d6, a moment M with a magnitude of d6*Fx acts on the worm shaft 6. Accordingly, in a case where a distance between the meshing portion and the oscillation center o relating to the axial direction of the worm shaft 6 is L6, a force Fr with a magnitude of M/L6 based on the moment M acts in the radial direction of the worm shaft 6 (upward direction in FIG. 12 and downward direction in FIG. 13). This force Fr is opposite in direction in the case illustrated in FIG. 12 and in the case illustrated in FIG. 13. Accordingly, a magnitude of an actual force Fy′ in a y direction in which the moment M is taken into account and acting from the worm wheel 4 to the worm shaft 6 in the meshing portion decreases in a case where the worm wheel 4 rotates in one direction as illustrated in FIG. 12 (Fy′=Fy−Fr) and increases in a case where the worm wheel 4 rotates in the other direction as illustrated in FIG. 13 (Fy′=Fy+Fr). Accordingly, a resultant force F′ of the actual meshing components of force in the y and z directions acting on the meshing portion decreases as illustrated by an arrow C in FIG. 14 in a case where the worm wheel 4 rotates in one direction and increases as illustrated by an arrow D in FIG. 14 in a case where the worm wheel 4 rotates in the other direction. As is apparent from the direction of the resultant force F′, the meshing reaction force that is applied from the meshing portion to the worm shaft 6 includes the components relating to the meshing direction of the worm wheel 4 and the worm teeth 5 (vertical direction in FIGS. 12 to 14) and the direction perpendicular to the axial direction of the worm shaft 6 {front and rear direction in FIGS. 12A and 13A and lateral direction in FIGS. 12B, 13B, and 14} regardless of the direction of the rotation of the worm wheel 4.
Accordingly, in the case of the electric power steering device according to the prior art described above, the worm teeth 5 is displaced in the perpendicular direction with respect to the worm wheel 4 based on the component of the meshing reaction force in the perpendicular direction (front and rear direction in FIG. 11) when the meshing reaction force is applied from the meshing portion to the worm shaft 6. Accordingly, when the rotational vibration is applied from the wheel side to the steering shaft 2, the worm teeth 5 might vibrate in the perpendicular direction in the meshing portion and the jarring rattling noise might be generated.